Metal hexafluorosilicates as catalysts in the production of polyesters



United States Patent 3,395,127 METAL HEXAFLUOROSILICATES AS CATALYSTS INTHE PRODUCTION OF POLYESTERS Frank Dobinson, Chapel Hill, NC, assignorto Monsanto Company, St. Louis, Mo., a corporation of Delaware NoDrawing. Filed May 5, 1964, Ser. No. 365,171 11 Claims. (Cl. 260--75)This invention relates to an improved method for the preparation of highmolecular weight polyesters, such as those obtained by condensationreactions of polyhydric alcohol and dibasic acids or reactivederivatives thereof, in the presence of a catalyst. More particularly,the invention relates to the use of metal hexafluorosilicates ascatalysts in the preparation of fiber-forming polyesters.

In the present day commercial manufacture of high polymericpolymethylene terephthalates it is common practice to use as startingmaterials a dialkyl terep-hth-alate and a glycol, for example, dimethylterephthalate and ethylene glycol. However, provided terephthalic acidof a sufficient degree of purity can be obtained, the dimethylterephthalate can be replaced by terephthalic acid. The terephthalicacid or the dialkyl ester thereof is subjected to esteritication orester-interchange with the glycol, preferably in the presence of asuitable catalyst and if desired under swperatmospheric pressureconditions. The bishydroxyalkyl terephthalate and low polymers thusformed are polycondensed, desirably under reduced pressure andpreferably in the presence of a polycondensation catalyst until afilament and fiber-forming high polymeric polymethylene terephthalate isobtained.

In the commercial preparation of polyesters, the use of apolycondensation catalyst to produce in short periods of time afiber-forming polymer of high molecular weight having a lack ofdiscoloration is very desirable. There have been many polycondensationcatalysts proposed for use in the preparation of polyesters. However,these prior art catalysts have been lacking in that either too long atime of polymerization is needed to produce a polymer having the desireddegree of polymerization or a discolored polymer is produced. Therefore,a catalyst that would enable the preparation of polyesters having a highmolecular weight and desirable color cmharacteristics in a relativelyshort period of time is desirable.

It is an object of this invention to provide an improved procas for thepolymerization of reactive intermediates obtained from glycol anddicarboxylic acids or polyesterforming derivatives thereof into highmolecular weight polyesters.

It is another object of this invention to provide a new and improvedprocess for producing polyethylene terephthalate through the use ofnovel catalysts that accelerate the polyester forming reactions.

It is a further object of this invention to provide a catalyzed processfor preparing polymeric polyesters which have excellent colorcharacteristics.

Other objects and advantages of this invention will be apparent from thedescription thereof which follows.

The objects of this invention are accomplished by conducting thepolymerization of the reactive intermediates obtained from glycols anddicarboxylic acids or polyesterforming derivatives thereof into highmolecular weight polyesters in the presence of catalytic amounts ofcertain metal hexafiuorosilicates.

The hexafluorosilicates of this invention are compounds of the formulaMSiF wherein M is a metal selected fro-m groups 2B, 7B and 8 of thePeriodic Chart of Elements. Illustrative of metal icehexafluorosilicates coming within the scope of the above structuralformula are those of the following metals: the group 28 metals includingzinc, cadmium, and mercury; the group 7B metals including manganese,technetium and rhenium; and the group 8 metals including iron, cobalt,nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum.The Periodic Chart of Elements referred to above and throughout thespecification and claims may be found on page 57 of Langes Handbook ofChemistry, 8th Ed. (1952).

The hexafluorosilicates of this invention may be prepared by reactingthe carbonate, oxide, acetate, or the like of the desired metal withhexafluorosilicic acid. The hexafiuorosilicic acid may be prepared bythe reaction of silicon tetrafluoride and hydrogen fluoride or by thereaction of silicon tetrafiuoride and water. The hexafiuorosilicatesused in this invention normally exist as hydrates and usually have sixmolecules of water attached to each hexafiuorosilicate molecule.However, this water will be driven off under the polymerizationconditions used to carry the present invention. Therefore, for thepurposes of the present invention, the hydrated as well as theunhydrated hexafluorosilicates may be used.

The amount of hexafluorosilicate employed should be from about 0.01 to0.5 percent by weight, based on the weight of dicarboxylic acid oresterforming derivative thereof used. It is preferred that the amount ofhexafiuorosilicate used be from about 0.03 to 0.1 percent by weight,based on the weight of dicarboxylic acid or ester-forming derivativethereof used. The particular hexafluorosilicate to be used as a catalystmay be added to the reaction at any point prior to the polymerizationstep.

The synthetic linear condensation polyesters contemplated in thepractice of the invention are those formed from dicarboxylic acids andglycols, and copolyesters or modifications of these polyesters andcopolyesters. In a highly polymerized condition, these polyesters andcopolyesters can be formed into filaments and the like and subsequentlyoriented permanently by cold drawing. The polyesters and copolyestersspecifically useful in the instant invention are those resulting fromheating one or more of the glycols of the series HO(CH OH, in which n isan integer from 2 to 10, with one or more dicarboxylic acids orester-forming derivatives thereof. Among the dicarboxylic acids andester-forming derivatives thereof useful in the present invention theremay be named terephthalic acid, isophthalic acid, sebacic acid, adipicacid, p-carboxyphenylacetic acid, succinic acid, p,p'-dicarboxybiphenyl,p,p-dicarboxycarbanilide, p,p-dicarboxythiocar banilide,p,p-dicarboxydiphenylsulfone, p-carboxyphenoxyacetic acid, pcarboxyphenoxypropionic acid, p-carboxyphenoxybutyric acid,p-carboxy-phenoxy-valeric acid, p-carboxyphenoxyhexanoic acid,p-carboxyphenoxy heptanoic acid, p.p-dicarboxydiphenylmethane,p,pdica'rboxydiphenylethane, p,p'-dicarboxydiphenylpropane,p,p'-dicarboxydiphenylbutane, p,p'-dicarboxydiphenylpentane,p,p'-dicarboxydiphenylhexane, p-,p'-dioarboxydiphenylheptane,p,p'-dicarboxydiphenyloctane, p,p-dicarboxydiphenoxyeth-ane, p,p'dicarboxydi-phenoxypropane, p,p'-dicarboxydiphenoxybutane,p,p'dicarboxydiphenoxypentane, p,p-dicarboxydiphenoxyhexane,3-alkyl4-(betacarboxy ethoxy) banzoic acid, oxalic acid, glutaric acid,pimelic acid, suberic acid, azelaic acid and the dioxy acids of ethylenedioxide having the general formula HOOC(CH OCH CH O (CH COOH wherein nis an integer from 1 to 4, and the alip'htic and cycloaliphatic arylesters and half esters, ammonium and amine salts, and the acid halidesof the above-named compounds and the like. Examples of the glycols whichmay be employed in practicing the instant invention are ethyleneglycols, trimethylene glycol, tetramethylene glycol and decamethyleneglycol, and the like. Polyethylene terephthalate, however, is preferredbecause of the ready availability of terephthalic acid and ethyleneglycol, from which it is made. It also has a relatively high meltingpoint of about 250 through 265 C. and this property is particularlydesirable in the manufacture of filaments in the textile industry.

Among the modified polyesters and copolyesters which are useful in thepractice of the instant invention are the polyesters and copolyestersmentioned above modified with chain terminating groups havinghydrophilic properties, such as the monofunctional ester-formingpolyesters bearing the general formula wherein R is an alkyl groupcontaining 1 to 18 carbon atoms or an aryl group containing 6 to 10carbon atoms, and m and n are integers from 2 to 22 and x is a wholenumber indicative of the degree of polymerization, that is, x is aninteger from 1 to 100 or greater. Examples of such compounds aremethoxypolye'thylene glycol, ethoxypolyethylene glycol,n-propoxypolyethylene glycol, isopropoxypolyethylene glycol,butoxypolyethylene glycol, phenoxypolyethylene glycol,methoxypolypropylene glycol, methoxypolybutylene glycol,phenoxypolypropylene glycol, phenoxypolybutylene glycol,methoxypolymethylene glycol, and the like. Suitable polyalkylvinylethers having one terminal hydroxyl group are the addition polymersprepared by the homopolymerization of alkylvinyl ethers wherein thealkyl group contains from 1 to 4 carbon atoms. Examples of suchchain-terminating agents are polymethylvinyl ether, polyethylvinylether, polypropylvinyl ether, polybutylvinyl ether, polyisobutylvinylether, and the like. The chain-terminating agents or compounds may beemployed in the preparation of the modified polyesters in amountsranging from 0.05 mole percent to 4.0 mole percent, based on the amountof dicarboxylic acid or dialkyl ester thereof employed in the reactionmixture. It is to be noted that when chain-terminating agents areemployed alone, i.e., without a chain-branching agent, the maximumamount that can be employed in the reaction mixture is 1.0 mole percent.Thus, unexpectedly, the addition of controlled amounts ofchain-branching agents along with the chain-terminating agents allowsthe introduction of an increased amount of the latter into the polymerchain than is otherwise possible when employing the chain-terminatingagents alone.

One will readily appreciate that the weight percent of chain-terminatingagent which may be employed in this invention will vary with themolecular weight of the agent. The range of average molecular weights ofthe chain-terminating agents suitable for use in this invention is from500 to 5000, with those agents having a molecular weight in the range of1000 to 3500 being preferred.

Materials suitable as chain-branching agents or crosslinking agents,which are employed to increase the viscosity or molecular weight of thepolyesters, are the poly- 015 which have a functionality greater thantwo, that is, they contain more than two functional groups, such ashyd-roxyl. Examples of suitable compounds are pentaerythritol; compoundshaving the formula:

wherein R is an alkylene group containing from 3 to 6 carbon atoms and nis an integer from 3 to 6, for example, glycerol, sorbitol, hexanetriol-1,2,6, and the like; compounds having the formula:

wherein R is an alkyl group containing from 2 to 6 carbon atoms, forexample, trimethylol ethane, trimethylol propane, and the like compoundsup to trimethylol hexane; and the compounds having the formula;

wherein n is an integer from 1 to 6. As examples of compounds having theabove formula there may be named trimethylol benzene-1,3,5, triethylolbenzene-1,3,5, tripropylol benzene-1,3,5 tributylol benzene-1,3,5, andthe like.

Aromatic polyfunctional acid esters may also be employed in thisinvention as chain-branching agents and particularly those having theformula o-o-rt" and in which R, R and R" are alkyl groups containing 1to 3 carbon atoms and R' is hydrogen or alkyl groups having 1 to 2carbon atoms. As examples of compounds having the above formula theremay be named trimethyltrimesate, tetramethyl pyrorne'llitatc,tetrarnethyl mellophonate, trimethyl hemimellitate, trimethyltrimellitate, tetramethyl prehnitate, and the like. In addition, theremay be employed mixtures of the above esters which are obtained inpractical synthesis. That is, in most instances, when preparing any ofthe compounds having the above formula, other related compounds havingthe same formula may be present in small amounts as impurities. Thisdoes not affect the compound as a chain-branching agent in thepreparation of the modified polyesters and copolyesters describedherein.

The chain-branching agents or cross-linking agents may be employed inthe preparation of the polyesters and copolyesters in amounts rangingfrom 0.05 mole percent to 2.4 mole percent, based on the amount ofdicarboxylic acid or dialkyl ester thereof employed in the reactionmixture. The preferred range of chain-branching agent for use in thepresent invention is from 0.1 to 1.0 mole percent.

In the practice of the present invention, the dibasic acid or esterthereof and the glycol are charged to the reaction vessel at thebeginning of the first stage of the esterification reaction and thereaction proceeds as in any wellknown esterification polymerization. Ifdesired, the calculated amounts of chain-terminating agent or chain-tenminating agent and chain-branching agent or cross-linking agent are alsocharged to the reaction vessel at this time.

When preparing the polyester from an ester, such as dimethylterephthalate, the first stage of reaction is carried out at about 180C. and at a pressure of 0 to 7 p.s.i.g. If the polyester is preparedfrom the acid, such as terephthalic acid, the first stage of reaction iscarried out at about 220260 C. and at a pressure of 15 to 60 p.s.i.g.The methanol or water evolved during the first stage of reaction iscontinuously removed by distillation. At the completion of the firststage, the excess glycol, if any, is distilled off prior to entering thesecond stage of the reaction.

In the second or polymerization stage, the reaction is conducted atreduced pressures and preferably in the presence of an inert gas, suchas nitrogen, in order to prevent oxidation. This can be accomplished bymaintaining a nitrogen blanket over the reactants, said nitrogencontaining less than 0.003 percent oxygen. For optimum results, apressure within the range of less than 1 mm. up to 5 mm. of mercury isemployed. This reduced pressure is necessary to remove the free ethyleneglycol that is formed during this stage of the reaction, the ethyleneglycol being volatilized under these conditions and removed from thesystem. The polymerization step is conducted at a temperature in therange of 220 to 300 C. This stage of the reaction may be effected eitherin the liquid, melt or solid phase. In the liquid phase, particularly,reduced pressures must be employed in order to remove the free ethyleneglycol which emerges from the polymer as a result of the condensationreaction.

Although the process of this invention may be conducted stepwise, it isparticularly adaptable for use in the continuous production ofpolyesters. In the preparation of the described polyesters, the firststage of the reaction takes place in approximately 2 1 to 2 hours. Theuse of an ester-interchange catalyst is desirable when starting withdimethyl terephthalate. In the absence of a catalyst, times up to 6hours may be necessary in order to complete this phase of the reaction.In the polymerization stage, a reaction time of approximately 1 to 4hours may be employed with a time of 1 to 3 hours being the optimum,depending on catalyst concentration, temperature, viscosity desired, andthe like.

The linear condensation polyesters, produced in accordance with thepresent invention, have specific viscosities in the order of about 0.25to 0.6, which represent the fiber-and filament-forming polymers. It isto be understood, of course, that non-fiber-forming polyesters may beproduced by means of the present invention, which have a greater or lessmelt viscosity than that reiterated above. For example, polyesters whichare useful in coating compositions, lacquers, and the like are withinthe scope of the present invention.

Specific viscosity, as employed herein, is represented by the formula:

Viscosity determinations of the polymer solutions and solvent are madeby allowing said solutions and solvent to fiow by force of gravity at 25C. through a capillary viscosity tube. In all determinations of thepolymer solution viscosities, a polymer containing 0.5 percent by weightof the polymer dissolved in a solvent mixture containing two parts byweight of phenol and one part by weight of 2,4,6-trichlorophenol, and0.5 percent by weight of water, based on the total weight of themixture, is employcd.

The polyesters of this invention may be produced to form filaments andfilms by melt spinning methods and can be extruded or drawn in themolten state to yield products that can be subsequently cold drawn totheextent of several hundred percent of their original lengths, wherebymolecularly oriented structures of high tenacity may be obtained. Thecondensation product can be cooled and comminuted followed by subsequentremelting and processing to form filaments, films, molded articles, andthe like.

Alternatively the polyesters of this invention may be processed toshaped objects by the wet spinning method, wherein the polyesters aredissolved in a suitable solvent and the resulting solution extrudedthrough a spinneret into a bath composed of a liquid that will extractthe solvent from the solution. As a result of this extraction, thepolyester is coagulated into filamentary material. The coagulatedmaterial is withdrawn from the bath and is then generally subjected to astretching operation in order to increase the tenacity and to inducemolecular orientation therein. Other treating and processing steps maybe given the oriented filaments.

If it is desired to produce shaped articles from the polyesters of thepresent invention which have a modified appearance or modifiedproperties, various agents may be added to the polyester prior to thefabrication of the articles or these agents may be incorporated with theinitial reactants. Such added agents might be plasticizers, antistaticagents, fire-retarding agents, stabilizers, and the like.

To further illustrate the present invention and the advantages thereof,the following specific examples are given, it being understood thatthese are merely intended to be illustrative and not limitative. Unlessotherwise indicated, all parts and percents are by weight.

The yarn color test employed in the examples consisted of determinationson the proximity to complete whiteness by reflectance measurements madeby using a spectrophotometer. The methods used were those recommended bythe Standard Observer and Coordinate System of the InternationalCommission on Illumination as fully set forth in the Handbook ofColorimetry, published in 1936 by the Technology Press, MassachusettsInstitute bif Technology. The results are to be interpreted asapproaching complete whiteness to the degree that the values givenapproach 100, which is taken as the complete whiteness value.

The specific viscosity values given in the examples Were determined atzero spinning times and were measured at about 25 C. in a mixture of133.5 grams of phenol, 66.8 grams of trichlorophenol, and 1 gram ofwater.

Example 1 A charge containing 200 grams of terephthalic acid, 400 ml. ofethylene glycol, 0.5 gram of calcium terephthalate, and 0.075 gram ofzinc hexafluotitanate was added directly to a polyester autoclave andthe system was purged 6 times with nitrogen, allowing the pressure torise to 150 p.s.i.g., and then releasing it slowly to atmosphericpressure each time. The maximum rate of heating was then applied to theclosed system, and when the temperature inside the bomb had reached toC., the stirrer was slowly started. When the temperature of the outsidewall of the autoclave had reached about 250 C. (the inside temperaturebeing about 230- 235 C. and the pressure being about 25 p.s.i.g.), theoff-vapor valve was adjusted to maintain these conditions of temperatureand pressure. As the first distillate containing water and some ethyleneglycol appeared, the esterification stage was considered to havestarted. The stirrer speed was set at rpm. The esterification step tookabout 40 minutes for completion, after which the pressure of the systemwas adjusted to atmospheric pressure. The heating rate was thenincreased until the temperature reached about 280 C. During this time,eXcess ethylene glycol was distilled off. An ethylene glycol slurry oftitanium dioxide was introduced through an injection port when theinside temperature had reached about 260 C. to 265 C. When the insidetemperature reached about 280 C., vacuum was applied. The insidetemperature was maintained at about 280 C. and the pressure wasmaintained at about 0.1 mm. Hg, and the polymerization continued until apolymer having a specific viscosity of 0.285 was obtained. Thepolymerization time for preparing the polymer was 23 minutes.

Fiber was spun from the polymer prepared above using conventional meltspinning procedures. However, the fiber was a dirty yellow color and hada whiteness value of less than 45.

This example shows the use of a known polyester polymerization catalystto produce a high molecular weight polymer in a short polymerizationtime. However, the resulting fiber had very poor whiteness.

Example 2 A polyester autoclave was charged with 200 grams ofterephthalic acid, 400 ml. of ethylene glycol, 0.1 gram of sodiumsulfate, and 0.165 gram of manganous acetylacetonate. Polyethyleneterephthalate having a specific viscosity of 0.305 was prepared fromthis mixture following the procedure of Example 1. The polymerizationtime for preparing the polymer was 105 minutes.

Fiber was spun from the polymer prepared above using conventional meltspinning procedures. The fiber was found to have a whiteness value of82.

This example shows the use of -a known polyester polymerization catalystto produce a high molecular weight polymer having good whiteness.However, the polymerization time necessary to produce the polymer wasextremely long.

Example 3 A polyester autoclave was charged with 200 grams ofterephthalic acid, 400 ml. of ethylene glycol, 0.1 gram of sodiumsulfate, and 0.1365 gram of zinc hexafiuorosilicate. Polyethyleneterephthalate having a specific viscosity of 0.321 was prepared fromthis mixture following the procedure of Example 1. The polymerizationtime for preparing the polymer was 38 minutes.

Fiber was spun from the polymer prepared above using conventional meltspinning procedures. This fiber was found to have a whiteness value of85.

Example 4 Example 5 A polyester autoclave was charged with 200 grams ofterephthalic acid, ml. of ethylene glycol, 0.1 gram of sodium sulfate,and 0.132 gram of cobalt hexafluorosilicate. Polyethylene terephthalatehaving a specific viscosity of 0.32 was prepared from this mixturefollowing the procedure of Example 1. The polymerization time forpreparing the polymer was 55 minutes.

Fiber was spun from the polymer prepared above using conventional meltspinning procedures. This fiber was found to have a whiteness value of95.

Example 6 Polyethylene terephthalate may be produced following theprocedure of Example using manganese hexafluorosilicate as a catalyst inplace of the cobalt hexafluorosilicate. Excellent fibers and filamentsmay be prepared from the polymer using conventional melt spinningprocedures.

It is to be understood that changes and variations may be made in thepresent invention without departing from the spirit and scope thereof asdefined in the appended claims.

What is claimed is:

1. In a process for producing synthetic, highly polymeric polyesterswherein a compound selected from the group consisting of dicarboxylicacids and ester-forming derivatives thereof and a molar excess of apolymethylene glycol having the formula, HO(CH OH, wherein n is aninteger from 2 to are reacted under polyesterification conditions andthe reaction is continued until a highly polymeric product is formed,the improvement which comprises carrying out the polymerization step inthe presence of a catalytic amount of a hexafluorosilicate selected froma group consisting of compounds of the formula MSiF wherein M is a metalselected from group 8 of the Periodic Chart of Elements.

2. In a process for producing synthetic, highly polymeric polyesterswherein a compound selected from the group consisting of dicarboxylicacids and ester-forming derivatives thereof and a molar excess of apolymethylene glycol having the formula, HO(CH ),,OH, wherein n is aninteger from 2 to 10 are reacted under polyesterification conditions andthe reaction is continued until a highly polymeric product is formed,the improvement which comprises carrying out the polymerization step inthe presence of from about 0.01 to 0.5 percent by weight, based on theweight of the selected compound, of a hexafluorosilicate selected fromthe group consisting of compounds of the formula MSiF wherein M is ametal selected from group 8 of the Periodic Chart of Elements.

3. In a process for producing synthetic, highly polymeric polyesterswherein a compound selected from the group consisting of dicarboxylicacids and ester-forming derivatives thereof and a molar excess of apolymethylene glycol having the formula, HO(CH OH, wherein n is ininteger from 2 to 10 are reacted under polyesterification conditions andthe reaction is continued until a highly polymeric product is formed,the improvement which comprises carrying out the polymerization step inthe presence of from about 0.03 to 0.1 percent by weight, based on theweight of the selected compound, of a hexafluorosilicate selected fromthe group consisting of compounds of the formula MSiF wherein M is ametal selected from group 8 of the Periodic Chart of Elements.

4. In a process for producing highly polymeric polyethyleneterephthalate wherein a compound selected from the group consisting ofterephthalic acid and ester-forming derivatives thereof and a molarexcess of ethylene glycol are reacted under polyesterificationconditions and the reaction is continued until a highly polymericproduct is formed, the improvement which comprises carrying out thepolymerization step in the presence of from about 0.03 to 0.1 per-centby weight, based on the weight of the selected compound, of ahexafiuorosilicate selected from the group consisting of compounds ofthe formula MsiF wherein M is a metal selected from group 8 of thePeriodic Chart of Elements.

5. The process as defined in claim 4 wherein the hexafluorosilicate usedis cobalt hexafiuorosilicate.

6. In a process for the production of a synthetic, highly polymericpolyester which comprises the steps of forming a reaction mixture of acompound selected from the group consisting of dicarboxylic acids andester-forming derivatives thereof and a molar excess of a polymethyleneglycol having the formula, HO(CH OH, wherein n is an integer from 2 to10, heating said reaction mixture to a elevated temperature sufficientto induce the reaction between the selected compound and the glycol,maintaining the reaction mixture at the elevated temperature until thereaction is substantially completed, continuously removing thelay-products by distillation as they are formed during the reaction,then raising the temperature of the reaction mixture in a range of abovethe boiling point of the glycol to remove excess glycol in the reactionmixture, and maintaining the reaction mixture at an elevated temperatureand under a reduced pressure until a highly polymeric product is formed,the improvement which comprises carrying out the polymerization step inthe presence of a hexafiuorosilicate selected from the group consistingof compounds of the formula Msiis wherein M is a metal selected fromgroup 8 of the Periodic Chart of Elements.

7. In a process for production of a synthetic, highly polymericpolyester which comprises the steps of forming a reaction mixture of acompound selected from the group consisting of dicarboxylic acids andester-forming derivatives thereof and from about 2 to 20 moles per moleof the selected compound of a polymethylene glycol having the formula,HO(Cl-l Ol-l, wherein n is an integer from 2 to 10, heating saidreaction mixture to a elevated temperature sufiicien-t to induce thereaction between the selected compound and the glycol, maintaining thereaction mixture at the elevated temperature until the reaction issubstantially completed, continuously removing the by-products bydistillation as they are formed during the reaction, then raising thetemperature of the reaction mixture in a range above the boiling pointof said glycol to remove excess glycol in the reaction mixture, andmaintaining the reaction mixture at an elevated temperature and under areduced pressure until a highly polymeric product is formed, theimprovement which comprises carrying out the polymerization step in thepresence of from about 0.01 to 0.5 percent by weight, based on theweight of the selected compound, of a hexafluorosilicate selected fromthe group consisting of compounds of the formula MSiF wherein M is ametal selected from group 8 of the Periodic Chart of Elements.

8. In a process for production of a synthetic, highly polymericpolyester which comprises the steps of forming a reaction mixture of acompound selected from the group consisting of dicarboxylic acids andester-forming derivatives thereof and from about 2 to moles per mole ofthe selected compound of a polymethylene glycol having the formula HO(CHOH, wherein n is an integer from 2 to 10, heating said reaction mixtureto an elevated temperature suflicient to induce the reaction between theselected compound and the glycol, maintaining the reaction mixture atthe elevated temperature until the reaction is substantially completed,continuously removing the byproducts by distillation as they are formedduring the re action, then raising the temperature of the reactionmixture in a range above the boiling point of said glycol to removeexcess glycol in the reaction mixture, and maintaining the reactionmixture at an elevated tempera-ture and under a reduced pressure until ahighly polymeric product is formed, the improvement which comprisescarrying out the polymerization step in the presence of from about 0.03to 0.1 percent by weight, based on the weight of the selected compound,of a hexafiuorosilicate selected from the group consisting of compoundsof the formula MSiF wherein M is a metal selected from group 8 of thePeriodic Chart of Elements.

9. In a process for production of a synthetic, highly polymericpolyester which comprises the steps of forming a reaction mixture of amember selected from the group consisting of dicarboxylic acids andester-forming derivatives thereof and from about 2 to 20 moles per moleof the selected compound of a polymethylene glycol having the formulaHO(CH OI-I, wherein n is an integer from 2 to 10, heating said reactionmixture to an elevated temperature sufiicient to induce the reactionbetween the selected compound and the glycol, maintaining the reactionmixture at the elevated temperature until the reaction is substantiallycompleted, continuously removing the by-products by distillation as theyare formed during the reaction, then raising the temperature of thereaction mixture in a range above the boiling point of said glycol toremove excess glycol in the reaction mixture, and maintaining thereaction mixture at an elevated temperature and under a reduced pressureuntil a synthetic polyester having fiber-forming properties is formed,the improvement which comprises carrying out the polymerization step inthe presence of from about 0.03 to 0.1 percent by weight, based on theweight of the selected compound, of a hexafluorosilicate selected fromthe group consisting of compounds of the formula MSiF wherein M is ametal selected from group 8 of the Periodic Chart of Elements.

10. In a process for the production of a highly polymeric polyethyleneterephthalate which comprises the steps of forming a reaction mixture ofa compound selected from the group consisting of terephthalic acid andester-forming derivatives thereof and from about 2 to 20 moles per moleof the selected compound of ethylene glycol, heating said reactionmixture to an elevated tempera-ture sufficient to induce the reactionbetween the selected compound and the glycol, maintaining the reactionmixture at the elevated temperature until the reaction is substantiallycompleted, continuously removing the by-products by distillation as theyare formed during the reaction, then raising the temperature of thereaction mixture in a range above the boiling point of the glycol toremove excess glycol in the reaction mixture, and maintaining thereaction mixture at an elevated temperature and under a reduced pressureuntil polyethylene terephthalate having fiber-forming properties isformed, the improvement which comprises carrying out the polymerizationstep in the presence of from about 0.03 to 0.1 percent by weight, basedon the weight of the selected compound, of a hexafiuorosilicate selectedfrom the group consisting of compounds of the formula MSiF wherein M isa metal selected from group 8 of the Perlodic Chart of Elements.

11. The process as defined in claim 10 wherein the hexafiuorosilicattused is cobalt hexafluorosilicate.

References Cited UNITED STATES PATENTS 3,228,913 1/1966 Nesty et al.

WILLIAM H. SHORT, Primary Examiner.

L. P. QUAST, Assistant Examiner.

1. IN A PROCESS FOR PRODUCING SYNTHETIC, HIGHLY POLYMERIC POLYESTERSWHEREIN A COMPOUND SELECTED FROM THE GROUP CONSISTING OF DICARBOXYLICACIDS AND ESTER-FORMING DERIVATIVES THEREOF AND A MOLAR EXCESS OF APOLYMETHYLENE GLYCOL HAVING A FORMULA, HO(CH2)NOH, WHEREIN N IS ANINTEGER FROM 2 TO 10 ARE REACTED UNDER POLYESTERIFICATION CONDITIONS ANDTHE REACTION IS CONTINUED UNTIL A HIGHLY POLYMERIC PRODUCT IS FORMED,THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE POLYMERIZATION STEP INTHE PRESENCE OF A CATALYTIC AMOUNT OF A HEXAFLUOROSILICATE SELECTED FROMA GROUP CONSISTING OF COMPOUNDS OF THE FORMULA